1. Field of the Invention
The field of art to which this invention pertains is the solid-bed adsorptive separation of monosaccharides. More specifically, the invention relates to a process for separating a ketose from a mixture comprising a ketose and an aldose which process employs an adsorbent comprising a crystalline aluminosilicate which selectively adsorbs a ketose from the feed mixture.
2. Description of the Prior Art
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. As a few examples, a separation process disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491 uses a type A zeolite to separate normal paraffins from branched-chain paraffins and processes described in U.S. Pat. Nos. 3,265,750 and 3,510,423 use type X or type Y zeolites to separate olefinic hydrocarbons from paraffinic hydrocarbons. In addition to their use in processes for separating hydrocarbon types, X and Y zeolites have been employed in processes to separate individual hydrocarbon isomers. As a few examples, adsorbents comprising X and Z zeolites are used in the process described in U.S. Pat. No. 3,114,782 to separate alkyl-trisubstituted benzene isomers; in the process described in U.S. Pat. No. 3,864,416 to separate alkyl-tetrasubstituted monocyclic aromatic isomers; in the process described in U.S. Pat. No. 3,668,267 to separate specific alkyl-substituted naphthalenes. Because of the commercial importance of paraxylene, perhaps the more well-known and extensively used hydrocarbon isomer separation processes are those for separating para-xylene from a mixture of C.sub.8 aromatics. In processes described in U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020; 3,663,638; and 3,734,974 for example adsorbents comprising particular zeolites are used to separate paraxylene from feed mixtures comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers.
In contrast, our invention relates to the separation of nonhydrocarbons and more specifically to the separation of monosaccharides. We have discovered that adsorbents comprising certain zeolites containing one or more selected cations at the exchangeable cationic sites exhibit adsorptive selectivity for a ketose with respect to an aldose thereby making separation of a ketose from a mixture comprising a ketose and an aldose by solid-bed selective adsorption possible. In a specific embodiment our process is a process for separating fructose from a mixture comprising fructose and glucose.
Fructose is considered to be the most soluble and the sweetest of the sugars. Relative to sucrose having a sweetness of 1.0, fructose has a relative sweetness of about 1.4 while that of glucose is 0.7. The literature indicates that one of its uses in pure form is as a source of calories for patients who must be fed intervenously and that under conditions of stress such as surgery, starvation, and diabetes fructose administered intravenously is utilized normally whereas glucose is not. Other indicated advantages of fructose over glucose for intervenous feeding are a more adequate provision of calories as a result of less loss of sugar in the urine and a shorter infusion time (with consequently less discomfort to the patient), and a more rapid formation of liver glycogen. While fructose exists widely in nature the methods for isolating high-purity fructose are, however, more difficult than the primary method for obtaining high-purity glucose. High-purity glucose is readily manufactured from starch (which is made up exclusively of glucose units) by hydrolysis with mineral acids at elevated temperature followed by refining and crystallization of the hydrolyzate while one method of obtaining high-purity fructose on the other hand involves hydrolysis of sucrose, separation of fructose as an insoluble lime-fructose complex, liberation of fructose by acidification of the complex with acids that form insoluble calcium salts (such as carbonic or phosphoric acid), removal of cation and anion contaminants by means of cation- and anion-exchange resins, concentration of the resulting solution to a thick syrup in vacuo, and finally crystallization of fructose. Extensive studies have been made on the production of fructose by hydrolysis of fructose-bearing polysaccharides extracted from the Jerusalem artichoke. The Jerusalem artichoke is not a crop plant in the United States, however, and additionally the harvesting of the artichoke tubers (where the polysaccharides are stored) is a relatively costly and seasonal operation. Several methods of separating glucose from invert sugar, leaving fructose, have also been attempted, such as formation of insoluble benzidine derivatives of glucose and sodium chloride addition compounds of glucose, but these have not been practicable. Because of the difficulty in separating or concentrating fructose, solutions of fructose in combination with one or more other sugars are used to obtain the benefit of the higher sweetness of the fructose. Invert sugar solutions, which contain fructose and glucose, and "high fructose" corn syrup, which contains typically 40-45% fructose and 50-55% glucose as the principal sugars, are examples. Our invention offers an easier more direct process for separating fructose from a feed mixture containing fructose and glucose to obtain a product stream enriched in fructose and a product stream enriched in glucose. Both products can be used in confectionery and bakery products, in the canning of fruits and vegetables, in beverages and in other products requiring such sweeteners.